The Power of Motive
I find the Unforgivable Curses to be quite intriguing in their own morbid way. What makes them so unforgivable? Why didn’t Harrys righteous anger feed the Cruciatus Curse? I think the answers can be found, naturally, in the books.
What is the powerful bit of magic that Moody mentions in Harry Potter and the Goblet of Fire?
“Avada Kedavra’s a curse that needs a powerful bit of magic behind it — you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a nosebleed.” — Impostor Moody (pg. 217, GoF, American edition)
Why couldn’t Harry, who is a powerful wizard and could conjure a Patronus Charm in his third year at Hogwarts, inflict a sufficiently powerful Cruciatus Curse to cause Bellatrix Lestrange to shrivel up in pain? Of course, she herself gave the explanation:
Never used an Unforgivable Curse before, have you, boy? You need to mean them, Potter! You need to really want to cause pain — to enjoy it — righteous anger won’t hurt me for long. — Bellatrix Lestrange (pg. 810, OotP, American edition).
Based upon these two quotes, I think that the powerful bit of magic that “Moody” mentions is simply hatred. A hatred so great that it consumes your entire soul. A hatred so alive that it dominates your waking thoughts and haunts your dreams.
How could anyone enjoy causing such unfathomable pain on either a person or a mere spider unless they were utterly filled with hatred? Righteous anger didn’t cut it for Bellatrix. Why? Because righteous anger calls for justice, not revenge. Righteous anger is good, hate is evil. Harry Potter isn’t full of hate, but full of the essence that Voldemort, who is the epitome of hate, despises: love. Love is the opposite of hate, thus, Harry could not inflict the Cruciatus Curse upon Bellatrix because he did not hate her. He wanted to kill her, yes, but not for the sake of killing her. Not because he just wanted her dead for no good reason. He wanted justice.
The magic of hate is also apparent in the Avada Kedavra. You would have to totally, utterly, completely hate the person to utter those two words effectively. Not merely loathe a person like James did Snape or Harry loathes Malfoy, but utterly hate them with such force that you can think of nothing else. The thinking that I outlined for the Cruciatus Curse is the same for the Avada Kedavra.
Of course, some might not see where hatred has any relevance for the Imperious Curse. The hatred for this curse is different than for the others. This is more of an arrogant hate, I think. A hate where you delight in controlling another, delight in holding their life, their mind, their body in your hands, delight in making them do your bidding, delight in making them do the wretched things to further your own plans. In short, it is a delight in tyranny, which is just another form of hate.
The Imperius Curse is based upon the Hate of Freedom; the Cruciatus Curse is based upon the Hate of Man; the Avada Kedavra Curse is based upon the Hate of the Living.
That is why these curses are unforgivable. It is not because of their words, or what they do. It is because of their motive, the power behind them. The power of hate.